Magnetic thin films magnetized perpendicular to the plane of the film have many applications for memory and data storage technologies, e.g. magnetic hard disk drives, Magnetic Random Access Memories (MRAM) or magnetic domain wall devices.
Perpendicular magnetization relies on Perpendicular Magnetic Anisotropy (PMA), to overcome the magnetostatic shape anisotropy, the favored in-plane magnetization in thin film geometry.
Several physical phenomena can induce PMA, e.g. crystalline anisotropy, surface or interface anisotropy, and magnetoelastic anisotropy. Interfacial anisotropy occurs at an interface between an Oxide Layer (OL) (e.g. MgO) and a Ferromagnetic Layer (FML) (e.g. Fe, Co, CoFe or CoFeB), and is of particular technological importance. Indeed, this interface structure is widely used in MRAM devices, whose memory elements are based on magnetic tunnel junctions, each having two magnetic electrodes magnetized perpendicular to the plane of the Silicon wafer and separated by an oxide tunnel barrier.
In addition to the cited oxide and ferromagnetic layers, the magnetic tunnel junction (MTJ) structure can include a non-ferromagnetic metallic (ML), or seed layers, in a stacked structure. The simplest layered stack to create Perpendicular Magnetic Anisotropy (PMA) in one of the two magnetic electrodes of a Magnetic Tunnel Junction is to form a single ferromagnetic layer over a metallic layer, and then deposit an oxide layer over the ferromagnetic layer to give a stack designated from bottom to top ML/FML/OL, or in reverse order, OL/FML/ML.
Standard processes used in the semiconductor industry require heating wafers up to elevated temperatures as high at 400° C. for extended periods of time as long as several hours in an annealing process. Therefore MTJ devices constructed through semiconductor processes must withstand the temperature and time used in these standard processes without any degradation in magnetic and/or magneto-transport properties.
The Boltzmann Factor is the probability (p) in equation (1), that a thermal fluctuation causes a memory bit in an MTJ to flip between two stable states corresponding to a logical “0” and “1”. The thermal stability is related to the energy barrier between the two states (E), Boltzmann's constant (kB), and the absolute temperature (T) in equation (2).Boltzmann Factor=p(E)=e−Δ  (1)Thermal Stability factor=Δ=E/kBT  (2)
In the case of PMA, the energy barrier E depends on the magnetic anisotropy of the storage (i.e. free) layer. For a uniform magnetization reversal mechanism, the energy barrier E is proportional to the product of Keff·tFML where tFML is the thickness of the ferromagnetic layer. Keff is the effective anisotropy constant (having the dimension of an energy per unit volume).
Keff can be modeled as the sum of the interfacial anisotropy and shape anisotropy.Keff=Interfacial Anisotropy+Shape Anisotropy  (3)
Interfacial anisotropy is inherent in the material properties and is represented by a constant Ki (energy per unit surface) divided by the ferromagnetic layer film thickness. The shape anisotropy reduces the thermal stability and is modeled by equation (4),Shape Anisotropy=−2πMs2  (4)
where Ms is the saturation magnetization, and tFML is the ferromagnetic layer film thickness. Interfacial Anisotropy causes PMA and the shape anisotropy reduces the PMA. In summary . . . .Keff=Ki/tFML−2πMs2  (5)
Therefore from equation 5 the thermal stability should improve as the ferromagnetic layer tFML gets thinner. However, this model does not apply when tFML gets below a critical thickness. Experimentation finds that below the critical thickness, the ferromagnetic layer loses its magnetization due to imperfections and inter-diffusion with neighboring non-magnetic elements. Therefore the thermal stability reaches its maximum at the critical ferromagnetic thickness in a simple ML/FML/OL stack.
The simple PMA stack only provides weak PMA since there is a single OL/FML interface. The interfacial anisotropy (Ki) is not strong enough to sustain PMA for ferromagnetic layers thicker than ˜15 Angstroms. Moreover, there is significant inter-diffusion between the ferromagnetic layer and the base metallic layer that is tantalum for example. Inter-diffusion can cause the interface between the ferromagnetic and metallic layers to be a magnetically “dead” layer. As a result, the magnetic properties of the ferromagnetic layer are found to degrade when tFML<˜8 Angstroms. For this simple stack interface structure, the thermal stability at the critical ferromagnetic thickness is only ˜0.2 erg/cm2 and too small for practical applications.
An improved interface structure can be created by two OL/FML interfaces, layered in the form OL/FML/OL. This leads to higher PMA and enables the use of a thicker ferromagnetic layer. However, it is difficult to fabricate using oxidation to form the second oxide layer without also oxidizing the ferromagnetic layer. This leads to thick magnetically dead layers, loss of magnetization, and an increase of the resistance—area product of the Magnetic Tunnel Junction (MTJ).
Thus, an improved MTJ is needed with two oxide/FML interfaces to provide high PMA in the reference and free layers. Furthermore, oxidation to form the upper (second) OL must be better controlled to prevent undesirable oxidation of the FML and loss of PMA.